1. Field of the Invention
The present invention relates to a receiver that detects an approximate value of the power of a reception signal. Especially, the invention relates to a receiver that enables detection of the highly precise approximate value with a high speed.
2. Description of the Related Art
In, for example, a communication system, it is indispensable to detect the power (level) of a reception signal by a relevant receiver. As an example, in a mobile-station device that is equipped in a radio communication system, the powers of the signals that are received from a plurality of base-station devices that exist in the neighborhood thereof are detected and compared at all times with one another by the receiver. Thereby, the receiver recognizes a base-station device, the power of which signal is maximum, to be an optimum base-station device. According to this recognition, the receiver selects that base-station device as an opponent device for its communication.
The above-described detection of the power is ordinarily realized through the execution of an operation process that is made according to the reception signal. Such operation process will hereafter be explained in detail.
Incidentally, in the claims of this specification, a component I represents the absolute value of the component I, and a component Q represents the absolute value of the component Q. Also, in the other parts of this specification, in the detection (operation) of an approximate value of the power of a reception signal, a component I represents the absolute value of the component I, and a component Q represents the absolute value of the component Q.
First, assume that I represents the value of a component I of the reception signal (the absolute value of the component I); and Q represents the value of a component Q (the absolute value of the component Q). Then, theoretically, the power P of the reception signal is given by the following Equation No. 1. It is to be noted that the component I and component Q of the reception signal mean two digital de-modulated signals, the phase difference between which is 90 degrees. Each of these digital de-modulated signals is obtained by de-modulating, for example, the reception signal that has been subjected to orthogonal modulation.
[Equation No. 1]P=(I2+Q2)1/2  (1) 
However, when attempting to realize the operation process given as the above Equation No. 1 with the use of an actual digital circuit, because the operation process contains therein the self-multiplication operation of, for example, the I or Q, the number of digits necessary for execution of the operation inconveniently becomes twice as large. Namely, the number of the bits necessary in the digital circuit inconveniently becomes twice as large. For this reason, the circuit becomes large in scale, and in addition the compactness that is loaded upon, the circuit inconveniently becomes high in degree. Also, even when executing such operation process with the use of, for example, a DSP (Digital Signal Processor) or a CPU (Central Processing Unit), because there is the above-described self-multiplication operation, the amount of operation processing, the time length of operation processing, etc. inconveniently becomes very large.
On that account, an attempt to use not the strict theoretical equation shown in the above Equation No. 1, but an approximate equation for operating the power of the reception signal has hitherto been made. And, an attempt has thereby been made to use only an addition operation instead of the multiplication/addition operations shown in the Equation No. 1.
Here, an approximate equation that is frequently used is shown below as Equation No. 2. It is to be noted that P represents the power (here, an approximate value) of the reception signal; MAX (I, Q) represents a larger one of the value I and the value Q; and MIN (I, Q) represents a smaller one of the value I and the value Q.
[Equation No. 2]P=( 10/11)×MAX (|I|,|Q|)+( 5/11)×MIN (|I|,|Q|)  (2) 
Also, conventionally, the operation equation shown in the above Equation No. 2 is further approximated to one operation equation that is suitable to the execution of the operation process in a digital circuit. This approximate equation is shown as the following Equation No. 3.
[Equation No. 3]P=MAX (|I|,|Q|)+(½)×MIN (|I|,Q|)  (3) 
Also, in FIG. 5, illustration is made of an example of the procedure of the power approximate-operation process that is taken when the receiver makes its operation of an approximate value of the power of the reception signal by using the approximate equation shown as the Equation No. 3.
Namely, in the receiver, first, the component I and component Q of the reception signal are taken in as digital signals (step S21). The value I and the value Q are then compared with each other to determine which one of them is larger or smaller (step S22).
As a result of this, if the value I is larger, the value I is set to be c=I and the value Q is set to be d=Q (step S23). If the value Q is larger, the value as the approximate value of the power of the reception Q is set to be c=Q; and the value I is set to be d=I (step S24).
Next, in the receiver, the value that is obtained by shifting the value d by 1 bit rightward (i.e., the value that is obtained by multiplying the value d by ½) is set to be d′ (step S25). Then, the operation of (c+d′) is executed. The result of this operation is detected signal (step S26).
However, in the conventional receiver arranged to detect the approximate value of the power of the reception signal by using the one approximate equation shown, for example, by the above Equation No. 3 (the one that is obtained by being made further approximated to the preceding approximate equation), the error that is produced by that approximate equation is large in value. Therefore, the precision of this approximate value is low. Resultantly, there was the inconvenience that the quality of the communication became deteriorated.
As an example, in the mobile-station device that, as described above, is at all times detecting the power of the reception signal from each of the base-station devices that exist in the neighboring area upon that mobile-station device, because the error made from that detection is too large, the mobile-station device erroneously recognizes the base-station device that is not optimum as being an optimum base-station device. As a result, there was the possibility that the mobile-station device would select a base-station device that was not the optimum base-station device.